As the yield and efficiency of light emitting diode (LED) technology has improved, LED print bar (LPB) imagers have been developed and used for xerographic printing applications, in higher performance and higher quality applications. For yield reasons, optical performance and compactness, full width LPBs, i.e., LPBs spanning the entire cross process direction, are often made as multi-chip assemblies carefully assembled and focused in a housing with a SELFOC® lens array, i.e., a gradient index lens array or GRIN lens array, as shown in FIG. 1. For clarity, the housing has been omitted in FIG. 1. SELFOC® lens array 50 is arranged between multi-chip LED array assembly 52 and photoreceptor drum 54. It should be appreciated that although a photoreceptor drum is depicted in FIG. 1, other photosensitive surfaces may also be used in the foregoing arrangement, e.g., a photoreceptor belt. During xerographic printing, LED light 56 from array assembly 52 is focused on drum 54 via lens array 50. The “self-focusing” property of SELFOC® lenses is well known in the art and therefore not further described herein.
As shown in FIG. 2, SELFOC® lens array 50 may be formed from a plurality of gradient index lens 58 within housing 60. Housing 60 may include angled wall 62 which causes lenses 58 to align in two rows, wherein the second row is offset from the first row. In an embodiment, the longitudinal axis of each lens 58 in the second row is the aligned with the point of contact between two adjacent lenses 58 in the first row.
Due to the construction methods and characteristics of LEDs, LED chips and lenses, a LPB has imperfect imaging characteristics which can negatively impact print quality. For example, one source of imperfect imaging is the characteristics of the SELFOC® lenses with their limited depth of focus and collection of light through several individual lenslet rods in the SELFOC® array of lenses. Not only does the image become out of focus quickly along the axial direction of the array lenslets, the image also becomes blurred in unique ways due to separation of focal rays from the lenses contributing to the image at a given point. FIGS. 3-8 depict various out of focus conditions according to optical modeling which agree with actual lens performance.
Even though the power of individual LEDs may vary within a chip and between chips, the LPB output power can be corrected to an acceptable uniformity of illumination within a chip and between chips using internal stored non-volatile memory (NVM) correction values. This correction works well when the LPB is in focus and all spots have the same basic shape. However, the developed photoreceptor image as the spot focus changes may be problematic, see for example FIGS. 9-11, where cross process profiles are shown for a LED spot in focus and two types of defocus, respectively.
Depending on development threshold, halftone design, xerographic resolution, print quality required and several other factors, it may be necessary to hold the LED focus range to a very small value, e.g., +/−50 μm for typical LPBs. This requires precision tolerancing of mounting hardware, i.e., higher costs, or tedious manual set-up in manufacturing and possibly in field service replacement of LPBs or backer bar for the photoreceptor. Even with such measures, the optimum focus and print quality is not obtainable. The present disclosure addresses all these problems in a practical and cost effective method.